Tegra186: implement quasi power off (SC8) state

This patch adds support for the SC8 system power off state. This
state keeps the sensor subsystem powered ON while powering down
the remaining parts of the SoC. The CPUs and DRAM are powered down
as part of this state entry and perform a cold boot when exiting SC8.

Change-Id: Iba65c661a7fe077a0d696f114bab3b4595e19a0d
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>